This invention relates to optical communication systems and more particularly to an amplitude modulation (AM) modulator for such systems.
To date the bulk of the fiber optics development have been directed and motivated by the trunk communications problem of providing more megabits point to point for less money. Further development of fiber optic trunks can be expected to attempt to obtain higher bit rates, and longer repeater spacings as well as improved reliability and sophistication. However, high bit rate trunks, (i.e., greater than 30 megabits per second (mb/s)) have been a peculiarly commercial long lines application rather than a military one.
Rather than pursue the application of fiber optics to higher levels of the hierarchy, which in the military are either non-existent or carried by radio/satellite it would appear pertinent to start asking what impact fiber optics can make at lower levels such as the local loop. With the military this would include not only strategic and tactical voice and data subscribers, but also intra-vehicle communications, such as shipboard and aircraft. Since the ability to compete on a cost basis with conventional twisted pair is not obvious, except for limited special purpose applications, it seems the major thrust in this development will be techniques to reconfigure existing systems on a single wire data bus basis, simultaneously trading off switch costs and cost of many twisted pair lines against a single fiber and interface electronics. When this is done the inherently wider bandwidth potentials of the optical fiber begin to translate into economic advantage even at the lowest levels of the communication network.
The data bus architecture for direct subscriber interface has by no means been completely ignored in the last few years. Motivations for considering this approach have included: (1) lower cost per line, (2) flexible installation and expansion, (3) electromagnetic interference (EMI) and electromagnetic pulse (EMP), and (4) small size.
At low data rates, buses are commonly implemented in digital equipment. With higher data rates and longer paths, implementation of data buses requires that considerable care be exercised in maintaining a matched impedance at all points. Otherwise spurious reflections can render the system inoperative. Systems using both analog and digital data and a loop configuration have been implemented by a variety of companies. These systems have been proposed as offering solutions to problems plaguing operational systems, such as command report center, field army headquarters communications and shipboard internal communications.
Data bus systems based on fiber optics technology have been investigated mainly for airframe use. All systems to date have been based on the use of bundle optical fiber technology and a star configuration with all wires coming into a central node where crosstalk is deliberately introduced to allow separate users to talk with each other. Because of the launching conditions on fibers, reflections are minimal so such a system is possible. Star communication systems allow various users to be in contact with only one intervening node but require 2N wires for N users, have limited flexibility and pose synchronization difficulties that demand increased interface equipment complexity and under utilization of the available bandwidth of the optical fiber.
Fiber optic loop systems eliminate many of the disadvantages of the star communication systems but presently have two limitations. First of all, the only drop and insert capability that has been demonstrated has been by dissection of an optical fiber bundle and use of a mixing rod following the bundle, which is lossy and inflexible. Secondly, for a reasonable number of users the loss encountered at each junction rapidly adds up to more than the allowable attenuation for typical solid state light sources and state of the art light receivers. If these two difficulties can be solved, however, it will be possible to construct a system that can take full advantage of the bandwidth capability of the optical fiber, use a minimum number of optical fibers and simple interface electronics to allow optical fibers to be used in the local loop with economic and performance advantages.
During a study of intrusion techniques for optical fibers, it was determined that an optical fiber immersed in an acoustic field or wave of proper spatial orientation and frequency will scatter light out of the optical fiber. Subsequently some of the details of the interaction were investigated and the process proved to be a rather strong one. This can be expected quite simply because a sound wave (acoustic wave) and a light wave will interact strongly when they have the same wavelength. But the ratio of the velocity of sound in a solid to the velocity of light is on the order of 10.sup.5, so the frequency of the sound wave having the same wavelength as a light wave is 10.sup.5 less. Since the energy in a photon (light wave quantum) and in a phonon (sound wave quantum) is determined by its frequency, then a photon that is 10.sup.5 times more energetic than a phonon can be controlled by a phonon. The analogy to a triode is obvious. Practicability of gain will be determined by the cross section for the interaction and the intensity of the background thermal phonons, but even in lieu of gain a useful modulator results. For a single mode waveguide a simplified analysis predicts that 80 - 90% modulation of the optical beam occurs with about 2 watts of VHF (very high frequency) power acoustic waves. Similar results can be expected with multimode waveguides, but more care in shaping the acoustic wave or its drive waveform must be exercised.
An interesting thing about this result is that although it is quite easy to receive light from a single optical fiber without breaking the fiber by placing a detector and index matching liquid or plastic in contact with the fiber, there has been no way of inserting light into the optical fiber at any arbitrary position along its length without breaking it. One approach to this problem suggested was to insert a phase modulation on the light beam from a source at the input to the optical fiber by mechanically stretching the optical fiber and then providing coherent detection at the other end of the optical fiber. This allows a series of talkers to input information to the end of the optical fiber from any point along the fiber, but did not provide the ability to receive and talk at any point along the optical fiber. This was because coherent detection requires the spatial superposition of a reference carrier and the information character, and a good reference carrier was impossible to obtain. If however, an amplitude modulation is applied to the light beam in the optical fiber by allowing it to interact with an acoustic field, then there can be provided convenient reception by means of intensity detection of power in the optical fiber and convenient transmission.